1. Field of the Invention
The present invention relates to a driving-force distribution device that is mounted on a four-wheel-drive vehicle having a pair of right and left main drive wheels and a pair of right and left auxiliary drive wheels to distribute driving force of a driving source to the pair of right and left auxiliary drive wheels.
2. Description of the Related Art
As a conventional driving-force distribution device that is mounted on a four-wheel-drive vehicle having a pair of right and left main drive wheels and a pair of right and left auxiliary drive wheels to distribute driving force of a driving source to the pair of right and left auxiliary drive wheels, a driving-force distribution device described in Japanese Patent Application Publication No. 2015-129534 (JP 2015-129534 A) is known.
The driving-force distribution device described in JP 2015-129534 A includes: a differential mechanism that distributes driving force of a driving source transmitted via a propeller shaft to a pair of right and left auxiliary drive wheels in a manner to allow differential motion therebetween; and a clutch mechanism that is disposed between one auxiliary drive wheel among the pair of right and left auxiliary drive wheels and the differential mechanism and can adjust the driving force transmitted from the differential mechanism to the one auxiliary drive wheel. When this four-wheel-drive vehicle travels in a two-wheel-drive state, coupling between the propeller shaft and the driving source is disconnected and the clutch mechanism is released, which stops rotation of the propeller shaft and a differential case of the differential mechanism. In this case, a pair of pinion gears of the differential mechanism is supported via a pinion shaft supported by the differential case to rotate in opposite directions.
In the four-wheel-drive vehicle including the driving-force distribution device configured as described above, during traveling in the two-wheel-drive state, rotation of the propeller shaft and the differential case stops. Consequently, travel resistance due to rotational resistance associated with the rotation thereof decreases and fuel efficiency is improved.
In the driving-force distribution device described in JP 2015-129534 A, rotation of the pinion gears of the differential mechanism is lubricated with lubricant in the differential case. However, because rotation of the differential case stops during traveling in the two-wheel-drive state, the lubricant is not supplied to one of the pinion gears that is positioned above the rotation axis of the differential case when the angle of the pinion shaft in this stop state is nearly perpendicular to the horizontal direction. If this state continues for a long period of time, wear of the pinion gear may be accelerated due to lack of lubrication.
In order to prevent such lack of lubrication of the pinion gear, the clutch mechanism may be operated, for example, at predetermined time intervals so that the differential case and the propeller shaft can be rotated by rotational force of the auxiliary drive wheels. However, in this case, when the differential case and the propeller shaft start rotating, shock or vibration may occur and effect of improving the fuel efficiency may be limited.